It is a good thing oil leaks don’t cause cars to fail safety inspections (at least not here in the States.
Would you rather not have known about problems with your car? There is a very good chance that your car does suffer from all the issues the Dealer mechanics found.
The parameters you have listed (long trips,wanting to get another 100K out of the car) means fixing the car right (does not mean it must be the Dealer).
With the restrictions you have listed your only option about the oil loss is frequent checking and adding. I myself don’t take long trips in cars leaking significant amounts of oil.